Docs on plotutil functions should describe behavior on multiple calls

Original [issue 138](https://code.google.com/p/plotinum/issues/detail?id=138) created by eaburns on 2014-02-03T00:24:33.000Z:

For example, each call to plotutil.AddLines will restart the color pallet. If you want to cycle through the colors, you have to add all lines in a single call.